Unadulterated: The passage of the interstellar comet 2l / Borisov on our sun could have been the first star encounter of this chunk. Because it is more original than any comet known from our solar system. Astronomers conclude this from the strong polarization of the light it reflects – it speaks for the comet’s almost unweathered surface, as they report in the journal “Nature Communications”.
In August 2019, astronomers discovered the interstellar object 2I / Borisov – the less than a kilometer large chunk was just approaching the inner solar system. A short time later, observations showed that this extrasolar object must be a comet. Like some “domestic” comets, 2l / Borisov also survived its passage near the sun not unscathed: In March 2020 a piece of the ice lump broke off.
Polarization reveals the state
But before Borisov left our solar system slightly damaged, astronomers took the opportunity to learn more about its structure. A team led by Stefano Bagnulo from the Armagh Observatory in Ireland observed the comet in winter 2019/2020 with the FORS2 instrument on the Very Large Telescope (VLT) of the European Southern Observatory in Chile. This shows the polarization of the light reflected by the comet.
“The arrival of 2I / Borisov from interstellar space represents the first opportunity to study the composition of a comet from another planetary system,” explains co-author Ludmilla Kolokolova of the University of Maryland. “In this way we can check whether the material of this comet differs in any way from that of our domestic variants.”
Similarities to Hale-Bopp
And indeed: “The polarization of 2I / Borisov is quite different from what we observe with comets in our solar system,” report the astronomers. The light of the interstellar visitor showed a noticeably strong polarization. Typically, this is an indication of a very original, not yet weathered surface on comets that has been altered by star winds.
In the solar system, an object was first observed that at least comes close to the interstellar guest in this respect: Comet Hale-Bopp. “At the time it was measured in the 1990s, the Hale-Bopp polarimetric curve was steeper and taller than any other comet seen before,” explain Bagnulo and his colleagues. Astronomers therefore assume that Hale-Bopp first flew past our sun before his last passage.
The first completely unadulterated comet
But as the measurement data for 2I / Borisov showed, the interstellar comet has an even more extreme polarization curve than Hale-Bopp. The researchers conclude that this icy chunk could be even more original and unchanged than the native comet. “2I / Borisov could be the first truly unadulterated comet ever observed,” says Bagnulo.
For 2l / Borisov, the passage through the sun in December 2019 could even have been a premiere: “The comet 2I / Borisov has probably never flown closer to the sun or any other star before,” according to the astronomers. As a result, it has never been exposed to the star wind and its surface is hardly weathered. The comet probably still carries the molecules from which it was formed in its extrasolar environment.
Originated on the edge of his home system
The polarimetry also provided initial indications of the environment: “The fact that the two comets are remarkably similar to each other suggests that the composition of the environment in which 2I / Borisov originated is not so different Environment in the early solar system, ”says co-author Alberto Cellino from the Turin Astrophysical Observatory.
The interstellar comet could therefore have been formed on the icy outer edge of its home system. But where exactly it comes from is open. Similar to the interstellar asteroid Oumuamua, astronomers suspect that 2l / Borisov was thrown from its original orbit around an alien star by gravity turbulence or a collision. (Nature Communications, 2021; doi: 10.1038 / s41467-021-22000-x)
Quelle: European Southern Observatory (ESO)